Contributing to Icons

[danirabbit]

Problem

Currently docs are all developer facing, but there's not really anything for designers. We could use some docs for how to get started contributing with design

One specific way for folks to do that is with Icons.

Proposal

We should probably:

• Link to the HIG section on iconograpy: https://docs.elementary.io/hig/reference/iconography
• Talk about tooling, specifically installing Inkscape
• Differentiate between system icons which should almost always be fd.o icon spec names vs app icons which should almost always be shipped with apps

Prior Art (Optional)

https://github.com/elementary/icons/blob/master/CONTRIBUTING.md